Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
One particular LPP technique involves irradiating a target material droplet with one or more pre-pulse(s) followed by a main pulse. In this regard, CO2 lasers may present certain advantages as a drive laser producing “main” pulses in an LPP process. This may be especially true for certain target materials such as molten tin droplets. For example, one advantage may include the ability to produce a relatively high conversion efficiency e.g., the ratio of output EUV in-band power to drive laser input power.
In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a source element, such as xenon (Xe), tin (Sn) or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV. The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror”) is positioned at a distance from the plasma to collect, direct (and in some arrangements, focus) the light to an intermediate location, e.g., focal point. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer. In a typical setup, the EUV light must travel within the light source about 1-2 m from the plasma to the intermediate location, and as a consequence, it may be advantageous, in certain circumstances, to use gases in the light source chamber that have a relatively low absorptance of in-band EUV light. In one configuration, the laser beam may be focused through a central opening of the collector mirror onto a droplet target for plasma generation. The EUV radiation emitted in the backwards direction is then reflected at near-normal incidence by the collector mirror and directed to the intermediate location. One advantage of this configuration is that a relatively large mass collector shell with a large thermal load capacity and low deformation potential can be employed that can be controlled by thermal management from the mirror's backside.
For the above-described configuration, a collector mirror having a graded high-temperature-stable coating with interface-engineered multi-layers is typically employed to provide relatively high EUV reflectivity at varying angles of incidence. These near-normal incidence (NI) collector mirrors tend to exhibit good thermal load capacity as well as good image fidelity under high heat load. The multi-layer (ML) coating may also provide substantial spectral filtering of out-of-band (OOB) EUV radiation. In addition, the ML coating can be stacked or the number of layer periods can be increased providing sacrificial layers that extend the useful lifetime of the NI collector.
For EUV light sources designed for use in high volume manufacturing (HVM) environments, the lifetime of the collector mirror is a critical parameter affecting efficiency, downtime, and ultimately, cost. During operation, debris are generated as a by-product of the plasma which can degrade the collector mirror surface. These debris can be in the form of high-energy ions, neutral atoms and clusters of target material. Of these three types of debris, the most hazardous for the collector mirror coating is typically the ion flux. Generally, for the configuration described above, the amount of neutral atoms and clusters from the droplet target impinging onto the collector may be small since most of the target material moves in a direction pointing away from the collector surface, (i.e., in the direction of the laser beam). In the absence of debris mitigation and/or collector cleaning techniques, the deposition of target materials and contaminants, as well as sputtering of the collector multilayer coating and implantation of incident particles can reduce the reflectivity of the mirror substantially.
In more detail, the interaction of ions with energies of around a few kilo-electron volts with the surface results in erosion of the material of the MLM coating. In one study, an erosion rate of ±0.2 layers per million pulses was observed. This layer removal can be attributed to sputtering during impact of energetic particles emitted from the plasma. As indicated above, the collector mirror coating can include sacrificial layers and still provide full EUV reflectivity. Assuming an erosion rate of 0.2 layers/Mpulses and 500 sacrificial layers, an unprotected collector would only be useful for about 2.2×109 Pulses, which corresponds to a lifetime of about 2 days in a HVM environment at a repetition rate of 50 kHz.
With the above in mind, Applicants disclose a Laser Produced Plasma EUV Light Source with Ion-stopping Buffer Gas, and corresponding methods of use.